Measures of Oxygen and Oxygen Demand

Dissolved Oxygen

Oxygen in water is available to the plants and animals that live there only if it is dissolved. Dissolved oxygen or DO can range in concentration from 0 to 14.6 parts per million in water. This is also equivalent to a weight-based measure, milligrams per liter (or mg/l). The amount of oxygen that can be dissolved in water is inversely related to temperature - that is as the water temperature gets higher, the amount of oxygen that can be dissolved in the water goes down. It is also possible under some circumstances to have oxygen levels above 14.6 mg/l. This can happen where water goes over a dam or other structure that causes unusual amounts of mixing. The more oxygen that is in the water, the more diversity can be expected in the plants and animals found in the water.

Pollutants that make DO go down (besides heat) are any organic wastes such as animal or human sewage or any chemicals that will be decomposed by bacteria in the water. The growing bacteria that break down either the organic or chemical wastes consume oxygen for their reproduction and thus take oxygen out of the water and away from the other plants and animals.

Biochemical Oxygen Demand (BOD)

BOD is a lab test that measures the total amount of oxygen per unit volume of water required to bacterially oxidize (stabilize or break-down) the organic matter in the water.  Samples are incubated under standard conditions for periods of 5, 10, 20 or 30 days.  The standard test is for 5 days. The higher the BOD, the more oxygen depletion will take place.



Chemical Oxygen Demand (COD)

A similar test which measures how much oxygen that the oxidation of chemicals in the water might use.



Testing Information

Tests for these variables can be biased by the presence of toxic chemicals which kill the bacteria that would break down the organic materials or oxidizable chemicals. When measuring DO, care must be taken not to introduce extra air from the atmosphere as the sample is taken. All samples must be tested immediately or else iced down and taken back to the lab for testing. The ice slows down any on-going chemical and biological reactions.